1. Field of the Invention
This invention relates generally to maximizing radio frequency transmission power and minimizing phase-error in a wireless communication device transmitter. More particularly, to a transmitter architecture having a secondary phase-error correction loop including an amplitude reconstruction system.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards, to which these systems must conform, have evolved. For example, in the United States, third generation portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset xcfx80/4 differential quadrature phase shift keying (xcfx80/4-DQPSK) or variations thereof and the access format is TDMA. Other standards may require the use of, for example, CDMA.
Similarly, in Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrowband TDMA access environment.
Furthermore, in a typical GSM mobile communication system using narrowband TDMA technology, a GMSK modulation scheme supplies a very clean phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used thus allowing efficient modulation of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Other transmission standards, such as that employed in IS-136, use a modulation scheme where both a PM signal and an amplitude modulated (AM) signal are transmitted. Standards employing these schemes increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, even though it would be desirable to have one portable transceiver that can accommodate all of the above-mentioned transmission schemes, existing GSM modulation schemes are not easily adapted to transmit a signal that includes both a PM component and an AM component. One reason for this difficulty is that in order to transmit a distortion free signal containing a PM component and an AM component, a highly linear power amplifier is required. Unfortunately, highly linear power amplifiers are very inefficient, thus consuming significantly more power than a non-linear power amplifier and drastically reducing the life of the battery or other power source.
This condition is further complicated because transmitters typically employed in GSM communication systems transmit in bursts and must be able to control the ramp-up of the transmit power as well as have a high degree of control over the output power level over a wide power range. In GSM, this power control is typically performed using a closed feedback loop in which a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is fed back to the input of the power amplifier. Furthermore, in GSM systems, the transmitted signal typically has a constant power envelope, thereby making possible the use of a high efficiency (and therefore, non-linear) power amplifier. Further still, in these burst transmission systems in which the power amplifier output ramps up over a period of time, there is insufficient power amplifier output to provide phase-error correction feedback until the power amplifier can output sufficient power with which to feed back to a translation loop in the upconverter.
When attempting to include a PM component and an AM component in a GSM type modulation system, the power amplifier""s non-linearity could negatively affect the quality of the transmitted signal and introduce unrecoverable errors. Also, the transmitter""s non-linearity could cause intermodulation products and cause regrowth of the transmit spectrum, thereby causing an unacceptable adjacent channel power ratio. Furthermore, while attempting to include a PM component and an AM component in a GSM type modulation system, the power control loop will tend to fight against the amplitude variations present in the signal while attempting to maintain the desired output power. In such an arrangement, the power control loop tends to cancel the AM portion of the signal within its power control loop bandwidth.
In systems having transmit signals contain both PM and AM components, the output power can be controlled by setting a calibrated control signal on the power amplifier. Unfortunately, this requires the use of a highly linear, and therefore very inefficient, power amplifier. In non-burst transmission systems, the output power may be controlled by a feedback loop having a time-constant that is very low compared to the time-constant of the amplitude variations of the modulator. Another known method is to provide an open loop power control system, but in such a case the system has no control over the transmit power during the burst and the actual power level will likely vary over temperature, load conditions, aging, etc. Unfortunately, these methods are costly and inefficient.
Furthermore, in those transmission standards in which both a PM signal and an AM signal are sent to a power amplifier, unless the power amplifier is very linear, it may distort the combined transmission signal by causing undesirable AM to PM conversion. This conversion is detrimental to the transmit signal and can require the use of a costly and inefficient linear power amplifier.
With the increasing desirability of developing one worldwide portable communication standard, it would be desirable to allow portable transceivers to transmit a signal containing both a PM component and an AM component, while maximizing the efficiency of the power amplifier. Furthermore, as the GSM standard evolves further, such as with the development of enhanced data rates for GSM evolution (EDGE), it is desirable to have one portable transceiver that may operate in all systems.
The invention is a transmitter architecture having a secondary phase-error correction loop including amplitude reconstruction, that maximizes power amplifier efficiency and that compensates for phase-error caused by the power amplifier or any other component in the output path.
The invention maximizes the efficiency of a power amplifier and provides phase-error correction by incorporating a phase shifter in a secondary feedback path. During an initial portion of a transmit burst, an upconverter including a translation loop receives feedback only from a transmit voltage controlled oscillator (VCO). After the output of the power amplifier is sufficient to produce a feedback signal, the feedback signal is processed through a phase detector, the output of which is supplied as an error signal to a phase shifter. The phase detector determines a phase difference between the feedback signal taken from the output of the power amplifier and the input signal to the upconverter. The phase difference is supplied as an error signal from the phase detector to the phase shifter. The phase shifter adjusts the phase of the input signal and supplies a phase-corrected signal to the upconverter. In this manner, the phase detector and the phase shifter compensate for any phase-error introduced by the power amplifier. In order to introduce an AM signal into the transmit path, a control signal supplied to the power amplifier includes an AM signal.
The output signal from the power amplifier is directed through a variable gain element, which removes the AM portion of the power amplifier output. In this manner, the output of the power amplifier is supplied to the input of the upconverter, thereby allowing the translation loop to correct for phase shift induced not only by the VCO but also by the power amplifier. The AM portion of the signal is removed to avoid any additional AM-to-PM conversion in the feedback path.